1. Field of the Invention
The present invention relates to an organic light emitting display and a driving method thereof.
2. Discussion of Related Art
Recently, various flat panel display (FPD) devices having reduced weight and volume in comparison to a cathode ray tube (CRT), have been developed. FPD devices include a liquid crystal display, a field emission display, a plasma display panel and an organic light emitting display, etc.
The organic light emitting display displays an image using organic light emitting diodes (OLEDs) that generate light by recombination of electrons and holes.
The organic light emitting display as described above has various advantages such as an excellent color representation, a reduced thickness, etc. so that its market has been largely expanded to other applications such as personal digital assistant (PDA) and MP3 player, etc., besides cellular phone applications.
An OLED used in the organic light emitting display includes an anode electrode, a cathode electrode, and a light emitting layer formed therebetween. The OLED emits light from the light emitting layer, when a current flows from the anode electrode to the cathode electrode. The amount of emitted light according to the amount of current flowing in the OLED is varied to display various brightness levels.
FIG. 1 is a graph showing changes in saturation points according to changes in the amount of current flowing in an OLED. A horizontal axis of the graph shows the voltage of a ground power source connected to a cathode electrode of the OLED, and a vertical axis shows the amount of current flowing from an anode electrode to the cathode electrode.
Referring to FIG. 1, when the saturation current is 150 mA, the OLED operates in a saturation region when the cathode electrode has a voltage of 0V to −1V. When the saturation current is 200 mA, the OLED operates in a saturation region when the cathode electrode has a voltage of −1V to −2V. Also, when the saturation current is 250 mA, the OLED operates in a saturation region when the cathode electrode has a voltage below −2V.
In other words, the voltage of the cathode electrode varies according to the value of the saturation current. Therefore, the OLED is designed to emit light using a portion corresponding to the saturation current.
However, the voltage of the cathode electrode of an OLED in the organic light emitting display is generally set to a voltage corresponding to the case where the saturation current is the largest. In other words, although there are only a few images among all of the images displayed in the organic light emitting display are displayed at the highest gray level that require the largest saturation current, the voltage of the cathode electrode is set to a voltage corresponding to the case where the saturation current is the largest. Thereby, driving voltage is higher than necessary, and that causes an increase of power consumption.